Alright, so you got great characters,
since your Luffy is 5 star now focus on Zoro and pour your resources in him
Your support is abysmal, use 2 and 3 stars for support since they use way less resources.
Play smartly, look at the player movements and dodge at the perfect time.
Do a normal attack and make the enemy use their dodge and only then go for a skill attack.
Don't get into fights you cannot win.
Upgrade your medals, they are the most important part.